Hi Guest
It is impossible to tell a dogs age unless they are under 5 months (and they you can tell because they are teething).
Some dogs go grey at 4 or 5 years and others don't do this untill they are into their teens.
I met someone at the vet's yesterday with an eleven year old black GSD and she looked ancient - very arthritic, bent and grey but my nearly ten year old Dobe could pass for 5 or 6. (Body is fully mature but she has hardly a grey hair)
Christine